The purpose of the present study was to learn whether or not complicating changes in the caudate nucleus might account for inconsistent findings on the effects of prefrontal ablations on species-typical displays of gothic-type squirrel monkeys. Volumetric measurements of the corpus striatum revealed that a reduction in mass of the caudate nucleus may occur as a complication of surgical entrance of the ventricles. Such a complication may have contributed to the deficit in one of two monkeys that failed in the performance of displays following prefrontal ablations.